1. Field of the Invention
The present invention relates to a touch panel, particularly to an in-cell capacitive touch panel.
2. Description of the Related Art
The touch panel has been more and more popular recently. When touched by a finger or a stylus, a touch panel outputs an analog signal, and a controller converts the analog signal into a digital signal recognizable by a computer. The touch panel driver program processes the digital signal and controls a display card to present the touched position on the touch panel.
The current touch panels may be categorized into the resistive type, capacitive type, sonic type, infrared type and in-cell type. Among them, the in-cell type touch panel attracts much attention. The traditional resistive type or capacitive type touch panel needs an additional touch panel for detecting touch events on the display panel. The in-cell touch panel has the touch control function built inside the LCD cell without the additional touch panel and thus has advantages of lightweight, compactness and high optical performance. Therefore, the in-cell touch panel is highly valued.
At present, most of the in-cell touch panels belong to the optical sensation type, wherein photo sensors built inside the LCD cell sense brightness variation on the touch panel to detect whether there is a touch event. Refer to FIG. 1 and FIG. 2. The photo sensor may be a TFT (Thin Film Transistor) sensor 10, such as a photo sensor consisting of a photo TFT and a Readout TFT. The photo sensor may also be a p-i-n diode 12. However, the background of the detected image varies with the situation of the touch panel, and the environmental brightness affects the detection of the photo sensor. To solve such a problem, the readout system has to possess a dynamic feedback and auto-calibration capability. Thus, the system becomes more complicated. So far, the problem still lacks an effective solution.
Refer to FIG. 3a for a conventional in-cell capacitive touch panel. The in-cell capacitive touch panel has a plurality of sensing liquid crystal capacitors (Cslc) 14. Each sensing liquid crystal capacitor 14 is cascaded to a reference capacitor (Cref) 16. The capacitance variation of the sensing liquid-crystal capacitors 14 is used to detect touch events and determine touch points. Refer to FIG. 3b for the structure of the sensing liquid-crystal capacitor 14. The sensing liquid crystal capacitor 14 has an upper transparent substrate 141, an upper metal layer 142, a liquid crystal layer 143, a lower metal layer 144, and a lower transparent substrate 145 from top to bottom. The upper metal layer 142 functions as the electrode layer and provides a common voltage source (Vcom). The in-cell capacitive touch panel is exempted from the influence of environmental illumination and has a simpler readout system than the optical sensation type in-cell touch panel. However, the in-cell capacitive touch panel also has its own problems. For example, large-size in-cell capacitive touch panels are hard to fabricate because their capacitive sensors have pretty high parasitic capacitance. Further, the capacitive sensors can only reach a common level accuracy because of the high parasitic capacitance. Therefore, a high-resolution capacitive sensor is hard to achieve.
Accordingly, the present invention proposes a novel in-cell capacitive touch panel to solve the abovementioned problems.